The present invention relates to a sputtering target used as a magneto-optical recording material and made up of a rare earth metal-transition metal alloy as well as its fabrication, and regeneration processes.
An amorphous material made up of a rare earth metal-transition metal alloy such as Tb--Fe--Co is preferably used for a recording layer of a magneto-optical recording medium. Such a recording layer is usually formed by a magnetron sputtering technique. In the magnetron sputtering technique, it is required to leak a magnetic flux from a magnetic field generating means located on the back side of a target through the target out to the surface side of the target. When the permeability of the target is high, it is thus impossible to obtain sufficient magnetic field strength due to a leakage magnetic flux because the magnetic flux passing through the target increases. When a high-permeability target is used, therefore, it is required to make sure of a leakage magnetic flux by reducing the thickness of the target.
However, the thinner the target, the shorter the sputtering enabling time per target is. For this reason, the operation for opening a sputtering chamber for a target replacement and again drawing a vacuum therein must be frequently effected, resulting in productivity drops and, hence, some added cost. When a target product is produced by sintering, it is required to polish off an oxidized region in the vicinity of the surface of the product after sintering. The amount of the oxidized region to be polished is about 1 mm irrespective of target thickness; the thinner the target, the greater is the amount of the region making no contribution to sputtering and so rejected, again resulting in some added cost.
Regarding the target for forming a recording layer of magneto-optical recording media and its production process, for instance, some proposals have been made, as mentioned below.
(i) JP-B 64-2177 discloses a process of producing an alloy target material by melting a rare earth metal and a transition metal in an arc melting furnace into an alloy ingot, pulverizing the alloy ingot, and hot pressing the obtained alloy powders. Therein, the alloy ingot is subjected to wet fine grinding in an organic solvent. However, since the final powder particle size is as fine as 1 to 10 .mu.m as expressed by the mean particle diameter, the powders degrade with the progress of their oxidation. Therein, it is stated that the alloy ingot is produced by arc melting. In the arc melting, however, the raw material powders are melted by arc scanning, and so the molten metal is gradually cooled after passing through the arc, resulting in the precipitation of Fe. It is thus impossible to make the target thick due to its high permeability, resulting in a decrease in the service life per target. In the examples, targets of 1.2 to 1.3 mm in thickness are fabricated.
(ii) JP-B 1-19448 discloses a process of producing a target material comprising a mixed structure of an intermetallic compound of a rare earth metal with an iron group metal and a pure iron group metal by mixing the rare earth metal in a finely divided form with the iron group metal in a finely divided form, hot forming the obtained mixture at a temperature below the temperature at which a metal component system therein can be present as a liquid phase, thereby forming a dense compact, and heating the compact for a short period of time at a temperature higher than the temperature at which the metal component system in the compact can be present as a liquid phase. This target material contains 30 to 50% by weight of the rare earth metal with the balance being the iron group metal. Therein, it is alleged that the invention has the advantage of providing a high-strength target material for sputtering, which can form a magneto-optical recording medium having a stable composition and a low oxygen concentration.
However, the target material set forth in the aforesaid publication has a high permeability due to the presence of a phase comprising the pure iron group metal. It is thus impossible to make the target material thick, resulting in a decrease in the service life per target. In the examples, any target material having a thickness exceeding 2 mm is not fabricated at all. According to the process disclosed therein, it is required to pulverize the rare earth metal. However, the rare earth element has a ductility too high for pulverization, and so is likely to entrain oxygen therein upon pulverization. In the examples, indeed, the target material contains oxygen in not a little amount of, say, 1.5 at % or more.
(iii) JP-B 2-48623 discloses an alloy target comprising 10 to 50 at % of a rare earth metal with the balance being substantially a transition metal, and having a mixed structure comprising at least three phases, each being an intermetallic compound of the rare earth metal with the transition metal. This alloy target is fabricated by the sintering of alloy powders produced by a reducing diffusion technique. Therein, it is alleged that the invention can provide an alloy target having the advantages of being less likely to crack, having a dense and homogeneous composition and a sufficiently decreased oxygen content, being inexpensive, etc.
However, the presence of the three phases is tantamount to the presence of a phase having a higher transition metal concentration. The phase having a higher transition metal concentration, because of having a high permeability, renders it impossible to make a thick target, resulting a decrease in the service life per target. In the aforesaid publication, the ratio between the three phases is not directly disclosed. From the examples that do not show targets having a thickness exceeding 4.5 mm, however, it is believed that all the targets have a high permeability. Also, it is disclosed that the target is fabricated by producing an alloy powder comprising a single transition metal phase and an intermetallic compound surrounding and covering the same by the reducing diffusion technique, and sintering the alloy powder. With this process, it is difficult to cut down the cost of the raw material because it is impossible to recycle used-up targets by pulverization.
(iv) JP-B 3-6218 discloses a sintered composite target material comprising a rare earth metal grain, a transition metal grain, and a reaction diffusion layer of both. Therein, it is alleged that the advantages of the target material are large flexural strength, good impact resistance, low oxygen contents, and high sputtering rates.
However, the sintered composite target material set forth in the aforesaid publication, because of containing much transition metal grains, has an increased permeability. This renders it impossible to make the target material thick, resulting in a decrease in the service life per target. In the examples, indeed, any target material having a thickness exceeding 5 mm is not fabricated at all. In the production process set forth therein, the rare earth metal need be pulverized into rare earth metal particles used as the raw material. However, the rare earth metal has a ductility too high for pulverization, and so is likely to entrain oxygen therein upon pulverization.
(v) JP-B 3-54189 discloses a process of regenerating a used-up target by the filling-up of asperities on the surface of the target, which have been formed upon use with a magnetron sputtering system. Specifically, the asperities are filled up with supplementing powders composed of the same material as the target material. Then, the target is subjected to hot hydrostatic pressing or hot forging thereby forming a compact packed with the supplementing powders at high density. Subsequently, the target is heat treated to bond the target material firmly to the compact through diffusion. The thus regenerated target material is built up of a used-up target material portion, a powder compact portion and a diffusion bonded portion at a boundary therebetween. Therein, it is alleged that the regenerated target material has generally the same quality as a fresh target material.
However, the process set forth in the aforesaid publication is actually susceptible to a sputtered film composition variation ascribable to a composition variation of each portion.
One object of the invention is to decrease the permeability of an alloy target, which comprises a rare earth metal-transition metal alloy. Another object of the invention is to decrease the oxygen content of such an alloy target. Yet another object of the invention is to provide easy regeneration of an alloy target after it has been used up.